Figure 3-3:The background
shows the annual mean change of rainfall (color shading) for (a) the SRES
scenario A2 and (b) the SRES scenario B2. Both SRES scenarios show
the period 2071 to 2100 relative to the period 1961 to 1990, and were performed
by AOGCMs. Scenarios A2 and B2 are shown as no AOGCM runs were available
for the other SRES scenarios. The boxes show an analysis of inter-model
consistency in regional precipitation change. Regions are classified as
showing either agreement on increase with an average change of greater than
20% ( large increase), agreement on increase with an average change between
5 and 20% ( small increase), agreement on a change between -5 and +5% or
agreement with an average change between -5 and +5% ( no change), agreement
on decrease with an average change between -5 and -20% ( small decrease),
agreement on decrease with an average change of more than -20% ( large decrease),
or disagreement ( inconsistent sign). A consistent result from at least
seven of the nine models is defined as being necessary for agreement.

Projected climate change will have beneficial
and adverse environmental and socio-economic effects, but the larger the
changes and rate of change in climate, the more the adverse effects predominate.

3.16

The impacts of climate change will be
more severe the greater the cumulative emissions of greenhouse gases (medium
confidence). Climate change can have beneficial as well as adverse
effects, but adverse effects are projected to predominate for much of the
world. The various effects of climate change pose risks that increase with
global mean temperature. Many of these risks have been organized into five
reasons for concern: threats to endangered species and unique systems, damages
from extreme climate events, effects that fall most heavily on developing
countries and the poor within countries, global aggregate impacts, and large-scale
high-impact events (see Box 3-2
and Figure 3-1). The effects
of climate change on human health, ecosystems, food production, water resources,
small islands and low-lying coastal regions, and aggregate market activities
are summarized below. However, note that future changes in the frequency
or intensity of extreme events have not been taken into account in most
of these studies (see also Question 4).

Box 3-2 Concerns
about the risks from climate change rise with temperature.

Unique and threatened systems: Some changes in species
and systems have already been associated with observed changes
in climate, and some highly vulnerable species and systems may
be at risk of damage or even loss for very small changes in climate.
Greater warming would intensify the risks to these species and
systems, and place additional ones at risk.

Extreme climate events: Increased frequencies and intensities
of some extreme events have already been observed (see Question
2) and are likely to increase with further warming, as would
the risks to human life, property, crops, livestock, and ecosystems.
These risks increase where development is occurring in inherently
dynamic and unstable zones (e.g., river floodplains and low-lying
coastal regions) (see also Question
4).

Uneven distribution of impacts: In general, developing
countries are at greater risk of adverse impacts from climate
change than are developed countries, of which some of the latter
may experience market sector benefits for warming less than a
few °C. For greater warming, most regions are at risk of predominantly
negative effects from climate change. But developing countries
generally would continue to be more severely impacted than developed
countries. Within countries, ulnerability varies and the poorest
populations often have higher exposure to impacts that threaten
their lives and livelihoods.

Global aggregate impacts: Globally aggregated market
sector impacts may be positive or negative up to a few °C,
though the majority of people may be negatively affected. With
greater warming, the risk of negative global market sector impacts
increases, and impacts would be predominantly negative for most
people.

Large-scale, high-impact events: The probability of
large-scale, high-impact events within a 100- year time horizon
such as shutdown of the thermohaline circulation or collapse of
the West Antarctic ice sheet is very low for warming less than
a few °C. The risk, which is a product of the probabilities
of these events and the magnitude of their consequences, is largely
unquantified. For greater warming, and over a time horizon longer
than 100 years, the probabilities and the risks increase, but
by an amount that cannot now be estimated. See also Question
4.